An example of a fuel cell system is disclosed in JPA2000-315508 which generates power by electrochemical reactions of oxygen with a reformate gas produced by reforming fuel into a hydrogen-rich gas.
Carbon monoxide is contained in the reformate gas produced by the reformer which reforms fuel. Catalytic reactions in a carbon monoxide oxidizer are used in order to remove carbon monoxide before the reformate gas is supplied to the fuel cell due to the fact that the presence of carbon monoxide reduces power generation efficiency.
The temperature of the reformate gas immediately after being -produced in the reformer is approximately 300° C. Consequently, before the gas is supplied to the carbon monoxide oxidizer, it is required to reduce the temperature to approximately 100° C. by passing the gas through a heat exchanger. Catalytic reactions performed in the carbon monoxide oxidizer are performed efficiently only in a fixed temperature range about 100° C. The efficiency of reactions is reduced when the temperature of the reformate gas is either higher or lower than this fixed temperature range.
Furthermore it is necessary to homogenize the temperature distribution of the cooled reformate gas. When the temperature distribution of the reformate gas is not uniform, the overall reaction efficiency is reduced because areas of catalytic reactions having high efficiency exist together with the areas of low efficiency.
Although it is possible to homogenize the temperature distribution of the gas undergoing heat exchange by increasing the length of the heat exchanger core in the direction of gas flow, this increases the size of the heat exchanger.
Alternatively, although an agitator for gas in the downstream section of the heat exchanger has been proposed in order to homogenize the temperature distribution by mixing the gas, the provision of such a device is not preferable since it increases the complexity of the fuel cell system and increases manufacturing costs and the fuel cell size.